f test - Two Sample in Excel using the QI Macros

f test - two sample

In the constant quest to reduce variation and improve products, companies need to evaluate different alternatives. A f test using two samples compares two independent sets of test data. It helps determine if the variances are the same or different from each other. Consider the following example.

f test two sample example

If you're producing rubber made with two different recipes, you might want to know if the variances in tensile strengths are the same or different (Juran's QC Handbook 4th pg 23.74):

• The null hypotheis (H₀₎ - variances are the same
• The alternate hypothesis (H_a) - variances are different

Now, conduct a test and enter the data into Excel:

Select the data with the mouse and click on the QI Macros Menu to select the F test:

The QI Macros will prompt for a significance level (default = 0.05):

The f test macro will calculate the results:

Interpreting the F test results

Since F < Fcrit (3.05567 < 6.38823) and p value > a ( .15241 > 0.05), we can accept the null hypothesis that the variances are equal.

This may be a useful first step before running a t test.

Excel Note:
Notice that the F test reversed the order of the recipes. For some unknown reason, Excel requires that the recipe with the largest variance has to be first to ensure correct calculations. The QI Macros reorganizes your data to ensure that Excel performs the calculations correctly.

The f test is just one of the tests included in the QI Macros Statistical Process Control software. Other tests include:

• Levenes Test of variance for non-normal data
• t test assuming equal variances
• t test assuming unequal variances
• t test one sample
• ANOVA - Analysis of Variance
• Overvew of all hypothesis tests in Excel

Download the FREE 30-day Evaluation copy of the QI Macros Excel SPC Software for Six Sigma